linux-kernel - [PATCH] kernel:time fix race condition in alaramtimer.c

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Message-Id: <1373126870-17978-1-git-send-email-redmnic@gmail.com>
Date:	Sat,  6 Jul 2013 18:07:50 +0200
From:	Marcus Gelderie <redmnic@...il.com>
To:	artem.bityutskiy@...ux.intel.com, herbert@...dor.apana.org.au
Cc:	linux-kernel@...r.kernel.org, Marcus Gelderie <redmnic@...il.com>
Subject: [PATCH] kernel:time fix race condition in alaramtimer.c

This patch fixes a race condition whereby the process can be caused to
sleep indefinitely. The problem occurs when the process is preempted
after having set its state to TASK_INTERRUPTIBLE but before starting the
alarm.

Disabling preemption until the alarm has been set solves this problem.

A second problem may occur if the process reaches the alarm_cancel call
while still being in TASK_INTERRUPTIBLE. I do not see how this could
possibly happen (if schedule returns, the state is TASK_RUNNING; if
schedule is never called, the wakeup callback must have woken the
process (data==NULL) thereby setting its state to TASK_RUNNING) but
the code is clearly written under the assumption that it can happen.
In this case, the state must be set to TASK_RUNNING after the alarm
has been canceled for the same reasons as above.

Signed-off-by: Marcus Gelderie <redmnic@...il.com>
---
 kernel/time/alarmtimer.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
index f11d83b..81c8b31 100644
--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -585,15 +585,19 @@ static int alarmtimer_do_nsleep(struct alarm *alarm, ktime_t absexp)
 {
 	alarm->data = (void *)current;
 	do {
+		preempt_disable();
 		set_current_state(TASK_INTERRUPTIBLE);
 		alarm_start(alarm, absexp);
+		preempt_enable_no_resched();
 		if (likely(alarm->data))
 			schedule();

+		preempt_disable();
 		alarm_cancel(alarm);
+		__set_current_state(TASK_RUNNING);
+		preempt_enable_no_resched();
 	} while (alarm->data && !signal_pending(current));

-	__set_current_state(TASK_RUNNING);

 	return (alarm->data == NULL);
 }
-- 
1.8.1.5

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